Wireless radio or microwave frequency interrogators, for example radio frequency identification (RFID) interrogators or readers, may be used to read information from and/or write information to transponders, for example RFID transponders, commonly referred to as RFID tags.
RFID transponders or tags may store data in a wirelessly accessible memory, and may include a discrete power source (i.e., an active RFID tag), or may rely on power derived from an interrogation signal (i.e., a passive RFID tag). RFID readers typically emit a wireless interrogation or inquiry signal that causes the RFID transponder to respond with a return wireless signal encoding the data stored in the memory. The wireless signals typically have wavelengths falling in the radio or microwave portions of the electromagnetic spectrum. Whether radio or microwave frequencies are employed, such signals are commonly referred to as RF signals. Such a convention is adopted herein and throughout the attached claims.
Identification of an RFID transponder or tag generally depends on RF energy produced by a reader or interrogator arriving at the RFID transponder and returning to the reader. Multiple protocols exist for use with RFID transponders. These protocols may specify, among other things, particular frequency ranges, frequency channels, modulation schemes, security schemes, and/or data formats.
RFID transponders typically include a semiconductor device (e.g., a chip) and one or more conductive traces that form an antenna. The semiconductor device includes an integrated circuit that typically includes memory, logic circuitry and power circuitry. Typically, RFID transponders provide information stored in the memory in response to the RF interrogation signal received at the antenna from the interrogator or reader. Some RFID transponders include security measures, such as passwords and/or encryption. Many RFID transponders also permit information to be written or stored in the memory via an RF signal.
While RFID transponders provide various types of information stored in memory, the RFID transponders are presently incapable of transmitting their own coordinates of location to interrogators. Instead, techniques such as time difference of arrival (“TDOA”), time domain phase delay on arrival (“TD-PDOA”), spatial domain phase delay on arrival (“SD-PDOA”), and frequency domain phase delay on arrival (“FD-PDOA”) are used to calculate the distance between an interrogator and an RFID transponder. However, such techniques have to date typically been unsuccessful in multipath propagation environments. Multipath refers to reflections of a wireless signal that result in reception of the wireless signal at an antenna via two or more paths. Outdoor sources of multipath include the ground, the atmosphere, mountains and buildings, while indoor sources of multipath include floors, walls, ceilings, and metal objects. Sources of multipath introduce error in RFID transponder detection by affecting power and phase measurements, thereby distorting information that may be extracted from the RF signals by interrogators.
Conventional understanding is that each of these techniques is incapable of producing accurate distance measurements, at least in the unlicensed ISM (industrial, scientific and medical) band of 902-928 MHz commonly used by UHF RFID, while compensating for the effects of practical multipath interference.
For example, TDOA requires operating RFID interrogators and transponders in short pulse mode. UHF RFID is a very short range narrowband technology, with a typical transponder read range on the order of 10-20 feet. However, because the roundtrip signal delay for a range of 10-20 feet is on the order of a few tens of nanoseconds and the bandwidth is narrow, RFID interrogators and transponders cannot operate in the short pulse mode required by TDOA.
As another example, distance measurements made using phase-based techniques according to conventional approaches, such as with TD-PDOA, SD-PDOA, and FD-PDOA, are prone to significant error in multipath environments. Multipath interference causes multiple rays of a transmitted RFID signal to constructively and destructively interfere with the signal strength and the phase of the transmitted RFID signal. Accordingly, RFID interrogators attempting to measure distance by employing phase-based techniques in an indoor multipath environment may result in distance measurements with an error in excess of 300%.
New approaches for operating interrogators in multipath environments are desirable.